Deep hypogene oxidation of porphyry copper potassium-silicate protore at Butte, Montana; a theoretical evaluation of the copper remobilization hypothesis

Abstract

The oxidation effects of potassium-silicate protores from heated meteoric water have been thermodynamically computed and confirm previously described copper vein-forming mechanisms involving hypogene leaching and enrichment of chalcopyrite-bearing protore (Brimhall, 1979). Intense copper leaching of protore chalcopyrite is shown to be related to a state of peroxidation in which the only stable minerals are quartz, pyrite, and muscovite following the complete destruction of the wall-rock oxygen fugacity buffer assemblage including hematite dissolution. Because modification of the protore continues to higher fluid/rock ratios, copper is extracted from the fluid phase and again is fixed in the rock as sulfides, first as bornite and ultimately as chalcocite-covellite associated with the advanced argillic alteration assemblage. This end-stage assemblage should occur only where extremely high fluid/rock ratios are reached in zones of unusually high permeability near the source of the heat responsible for the fluid motion.The results of the calculations are interpreted to mean that certain intervals of fluid/rock ratio are characterized by net base metal leaching and other intervals by metal fixation. Attainment of only intermediate fluid/rock ratios may result in the major amount of the copper in a protore being remobilized into solution and leaving the system through ground-water circulation. In contrast, in highly evolved systems where the fluid/rock ratio has increased to extremely high values, the presence of an advanced argillic alteration assemblage with chalcocite and covellite strongly suggests the presence within a district of a partially oxidized, potassium-silicate, disseminated sulfide protore, including a strongly leached and sericitized quartz-pyrite protore relict.Near-surface vein mineralization may conceivably be related to primary or unoxidized protore at depth in two distinct ways, depending upon the origin of the intrusives and the fracture systems responsible for circulation of oxidizing meteoric water. First, late intrusives which are cogenetic with the magmatic heat source of the protore may remobilize ore-forming elements upon deep hypogene oxidation of pyritic disseminated sulfides. Second, much later, totally unrelated intrusive activity and deep crustal faulting, perhaps related to entirely different petrotectonic regimes, may provide access for meteoric water to penetrate much older protores, resulting in the formation of near-surface ore deposits by element recycling.It is therefore suggested that near-surface hydrothermal base and precious metal deposits be explored to considerable depths for possible low-grade protore potential.